The present invention is directed to methods for chemical shift imaging of a subject in a nuclear magnetic resonance imager, wherein the subject is positioned in both a static magnetic field and a varying gradient field and subjected to a sequence of RF pulses for exciting nuclear magnetic resonance in the subject.
Nuclear magnetic resonance devices are known in the art. In such devices, nuclear magnetic resonance (NMR) spectra are obtained from a subject which is positioned in a static magnetic field together with a varying gradient field, and which is subjected to a sequence of RF pulses. The use of the chemical shift imaging (CSI) technique in NMR imaging is also known, and has been described by A. A. Maudsley et al, Journal of Magnetic Resonance 51, 147 (1983) and by T. R. Brown et al, Proceedings of the National Academic Society, USA, 79, 3523 (1982).
The CSI technique provides both spectral and spatial resolution, and in vivo phosphorus spectroscopy is of specific interest for diagnostic purposes. However, due to the low intrinsic sensitivity of phosphorus spectroscopy the acquisition of localized phosphorus spectra using the CSI technique requires relatively long data acquisition times.
It has been shown (see P. Bachert-Baumann et al in Magnetic Resonance in Medicine, 15, pp. 165-172 (1990)) that the phosphorus signal intensity can be increased by saturation of the proton water spins surrounding a phosphorus atom in non-localized phosphorus spectroscopy. As described there, an MR system was equipped with two RF channels -- one for the resonance frequency of phosphorus, the other one for the resonance frequency of protons. The so-called WALTZ-8-supercycle as described by A. J Shaka et al in Journal of Magnetic Resonance 52, 335 (1983) was used for broad band decoupling of phosphorus and protons. The proton spins were irradiated during the acquisition period of the phosphorus MR signal or during a presaturation phase prior to the phosphorus excitation pulse. Signals enhancements were detected in sequences using decoupling as compared to sequences without decoupling. Signal intensity changes of resonance lines in double resonance experiments were attributed to the Nuclear Overhauser Effect. The magnetization of the proton affects the relaxation mechanism of the phosphorus spin system by bipolar interaction. The enhancement factor depends on the degree to which phosphorus spin relaxation is dominated by bipolar interaction with surrounding protons. The theoretical upper limit of enhancement is 124%.
Broad band proton decoupling in phosphorus NMR spectroscopy using the phase encoding method for spatial resolution has been carried out by P. Luyten et al in NMR in Biomedicine, Vol. 1, No. 5, 1989, pp. 177 -183. In the experiments therein described, proton decoupling was achieved by a standard WALTZ-4-sequence. The decoupling sequence was only applied during acquisition of the phosphorus NMR spectrum and was gated off during the recycle delay period. A repetition time of three seconds was used. An improvement in signal-to-noise ratio and spectral resolution was observed in the proton-decoupled phosphorus spectrum.